1. Field of the Invention
The present invention relates to a motor control circuit, and more specifically relates to a motor control circuit for apparatuses incorporating a motor rotating at a constant speed such as a hard disk drive, an optical disk drive, many other drive devices incorporating a motor and many office automation apparatuses incorporating a fan motor, which detects rotation speed of a motor to control the same and is able to stop the rotation of the motor rapidly when required, and further relates to a motor drive system using the same.
2. Description of Related Art
FIG. 3 is a conventional control circuit for a three phase motor which has been used for many kinds of office automation apparatuses.
The motor control circuit comprises a motor 9 including three windings 8a, 8b and 8c, a sensing circuit including Hall elements 1 as its major components, an input amplifier circuit 2 including differential amplifiers 2a as its major components, a reference signal generating circuit 3 which generates a signal determining a reference rotation speed of the motor 9, a torque control signal generating circuit 4 and a drive circuit 5. The torque control signal generating circuit 4 includes a speed comparing circuit 4a and an error amplifier circuit 4b. The drive circuit 5 includes a rotational direction changing-over circuit 5a which outputs a drive current for the motor 9 and stops the rotation of the motor 9 at an early stage by changing-over the rotational direction of the motor 9 by changing-over the drive current direction, and a drive current ceasing rotational number detection circuit 5b which detects the rotational number of the motor 9 for ceasing the drive current therefor and generates a signal for ceasing the drive current.
The operation of the motor control circuit is explained principally with reference to the signaling system in one phase of the three phases. First, the output signal from the Hall element 1, which detects the rotating condition of the motor 9, is applied to the input amplifier circuit 2 and is amplified at the differential amplifier 2a. Thereafter the amplified output is wave-shaped via a logic circuit, for example, and is outputted as a detection pulse P, and the detection pulse P is transmitted to the torque control signal generating circuit 4 and the drive circuit 5.
The reference signal generating circuit 3 produces a reference pulse K having a predetermined width and a phase which determines the rotation speed of the motor 9 and outputs the same. The speed comparing circuit 4a compares the detection pulse P with the reference pulse K, and further the amount of deviation of the periods or the phases based on the comparison result is integrated and the integrated value (normally in a form of voltage) is applied as an error to the error amplifier circuit 4a. The error amplifier circuit 4a is an amplifier which is constituted by a differential amplifier, for example, and uses the level of the torque control signal corresponding to the reference rotational number as a reference level, amplifies the error component with respect to the reference level for the torque control and adds the amplified error component to the reference level to generate a necessary torque control signal at the moment. Thereby, a torque control signal (normally in a form of voltage) T corresponding to the phase deviation amount between these pulses P and K is produced. The produced signal T is transmitted to a torque control terminal 5c in the drive circuit 5.
The drive circuit 5 receives the detection pulse P and produces a drive signal based on the received detection pulse P. At this instance, the drive circuit 5 further receives the torque control signal T, and produces and outputs a drive current which determines the amplitude of the drive signal (amount of current), namely the magnitude of torque, according to the magnitude (the voltage level) of the received signal T. Thereby, with the drive current the motor 9 is rotated in the direction to eliminate the error in the error amplifier circuit 4a. Thus, the rotation number of the motor 9 is controlled to settle at the reference rotational speed.
The above operation is explained further specifically. When the rotation speed of the motor 9 drops in comparison with the period of the reference pulse K which sets at the reference rotational speed, and the period of the detection pulse P lags, the drive circuit 5 outputs according to the torque control signal T to the motor 9 an output current which induces a larger torque than a reference torque value corresponding to a torque which balances with a load under the reference rotational speed. Contrarily, when the period of the detection pulse P advances, the drive circuit 5 applies according to the torque control signal T to the motor 9 a drive current which reduces the torque below the reference torque value or ceases the drive current. When the motor 9 is rotating at the set rotational number, under the condition of the reference rotational speed, the torque control signal T is maintained at a fixed level predetermined as a reference value.
Under a normal control, the torque control signal T is determined with reference to the detection pulse P of one phase as explained above, the drive currents for the three phase windings 8a, 8b and 8c are determined based on the detection pulse P and the determined currents are supplied to the motor 9 so as to control the rotational speed of the motor 9 to be constant.
When the drive circuit 5 receives a stop signal from an external device, the drive circuit 5 outputs a drive current which induces the maximum torque in the opposite direction and supplies the same to the windings 8a, 8b and 8c to thereby induce a braking force on the motor 9 in order that the rotational direction changing-over circuit 5a stops the rotation of the motor 9 as soon as possible. Due to the torque of the opposite direction the rotational speed of the motor 9 is decelerated to reduce the rotational speed. When the rotational speed reduces down to a predetermined speed, in other words reduces down to a predetermined rotational number, the drive current ceasing rotational number detection circuit 5b detects the speed reduction and the drive circuit 5 ceases the supply of the drive current to the windings 8a, 8b and 8c. Thereafter the rotation of the motor 9 is stopped through the frictional force. With the conventional motor control circuit the rotation of the motor 9 is stopped as thus explained.
When the timing of ceasing the drive current outputting of the opposite direction is further delayed from the timing when the rotational speed of the motor 9 reduces down to the predetermined speed, in other words, when the detection rotational number of the drive current ceasing rotational number detection circuit 5b is set at a further lower rotational number, the motor can be stopped in a further shorter time. However, if the detection rotational number is set at such a lower rotational number, there is a possibility that the motor rotates in the reversing direction with the conventional motor stopping method because the motor braking control is performed by adding the maximum torque of the opposite direction. Because of such possibility, the predetermined motor speed could not be lowered sufficiently such that it took a long time to stop the motor depending upon the frictional force. As a result, the time required for stopping the motor could not be shortened sufficiently.